A polarizing plate having light transmitting and screening functions is a basic constitutive element of display devices such as liquid crystal displays (LCD) as well as a liquid crystal having a light switching function. The fields of application of the LCD expand from small devices such as electronic calculators and watches in the early days to a wide variety of the fields such as notebook type personal computers, word processors, liquid crystal projectors, liquid crystal televisions, car navigations, and outdoor and indoor instruments, and they are used under wide conditions of low temperature to high temperature, low humidity to high humidity and low light volume to high light volume. Therefore, polarizing plates high in polarizing performance and excellent in durability are demanded.
At present, a polarizing film is produced in the following manner. That is, a substrate of polarizing film such as a stretched and orientated film of polyvinyl alcohol or a derivative thereof or a polyene type film obtained by dehydrochlorinating a polyvinyl chloride film or dehydrating a polyvinyl alcohol-based film to produce polyene and then orientating the film is dyed with iodine or a dichroic dye or is allowed to contain iodine or a dichroic dye as a polarizer, thereby obtaining a polarizing film. Of these polarizing films, iodine-based polarizing films which use iodine as the polarizer are superior in initial polarizing performance, but weak against water and heat, and have a problem in durability in case they are used for a long period of time in the state of high temperature and high humidity. In order to improve durability, there are proposed methods of treating with formalin or an aqueous solution containing boric acid or using a polymeric film of low moisture permeability as a protective film, but the improvement attained by these methods is not sufficient. On the other hand, dye-based polarizing films which use dichromatic dyes as polarizers are superior in moisture resistance and heat resistance to iodine-based polarizing films, but are generally insufficient in initial polarizing performance.
In the case of a direct vision polarizing film of neutral color (neutral grey absorbing all the wavelengths in the wavelength region of visible light region (400-700 nm)) comprising a polymeric film which has several kinds of dichroic dyes adsorbed thereto and orientated, if two pieces of the polarizing films are superposed one upon another so that the orientation directions cross at right angles (cross-nicol state) and if in this state there occurs leakage of light of a specific wavelength in the visible light region (color leakage), when these polarizing films are fitted to a liquid crystal panel, hue of the liquid crystal display sometimes changes in dark state. In order to inhibit change of color of liquid crystal display caused by color leakage of a specific wavelength in dark state upon fitting the polarizing films to a liquid crystal display device, it is necessary to uniformly reduce the transmittance in a cross-nicol state (cross-nicol transmittance) in wavelength region of the visible light region of the above polarizing films of neutral color comprising a polymeric film which has several kinds of dichroic dyes adsorbed and orientated.
In the case of a color liquid crystal projection type display, namely, a color liquid crystal projector, a polarizing plate is used in the liquid crystal image forming part, and formerly an iodine-based polarizing plate which is satisfactory in polarizing performance and shows neutral grey has been used. However, as mentioned above, the iodine-based polarizing plate suffers from the problems that it is insufficient in light resistance, heat resistance and moist heat resistance because iodine is used as a polarizer. For solving these problems, polarizing plates of neutral grey using dichroic dyestuffs as the polarizer are used, but in the case of polarizing plates of neutral grey, for averagely improving transmittance and polarizing performance in the whole visible light wavelength region, generally dyestuffs of three primary colors are used in combination. Therefore, there are problems that the light transmittance is low for satisfying the demand of market to make brighter the color liquid crystal projectors, and thus, for making brighter, the intensity of light source must be further enhanced. In order to solve this problem, three polarizing plates corresponding to three primary colors, namely, for blue color channel, green color channel and red color channel, have been used.
However, since the light is highly absorbed by the polarizing plate and images of small area of 0.9-6 inches are enlarged to about several ten inches to one hundred and several ten inches, reduction of brightness is unavoidable, and hence a light source of high luminance is used. In addition, demand for further improvement in brightness of liquid crystal projector is strong, and, as a result, the intensity of light source used is naturally further enhanced, thereby resulting in increase of light and heat applied to the polarizing plate.
As the dyes used for production of the above-mentioned dye-based polarizing plates, mention may be made of water-soluble azo compounds disclosed, for example, in the following Patent Document 1 to Patent Document 4.
However, the conventional polarizing plates containing the water-soluble dyes do not sufficiently satisfy the needs of market from the viewpoints of polarization characteristics, absorption wavelength region, hue, etc. Furthermore, the three polarizing plates corresponding to three primary colors, namely, for blue color channel, green color channel and red color channel in color liquid crystal projectors are not satisfactory in all of brightness, polarizing performance, durability under high temperature and high humidity conditions and light resistance in long-term exposure, and hence improvement in these points have been desired.    Patent Document 1: JP-A-2001-33627    Patent Document 2: JP-A-2002-296417    Patent Document 3: JP-A-2002-105348    Patent Document 4: JP-A-10-259311    Patent Document 5: JP-A-59-145255